Fix Content-Type detection if parameters are present

According to Media Type defininition in RFC 2616 [1], type/subtype
can be followed by any kind of parameters. The usage of a slash
character in a parameter is not forbidden.

This change uses ";" as delimiter as described and ignores correctly
any present parameters. The following Content-Type will now be
successfully detected:

    application/json
    application/json; charset=utf-8
    application/json; schema="prometheus/telemetry"; version=0.0.2

[1]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7